This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Pediatric Hydroxyurea Phase III Clinical Trial;a Randomized Double-blind Placebo Controlled Trial of Hydroxyurea Therapy in Very Young Children. The study will be preceded by a two-year Internal Pilot Study. All the children enrolled in the Internal Pilot Study and the main study will receive study treatment for the duration of 104 weeks. All children in the Internal Pilot Study will be followed until the last child&&#35;39;s study treatment has ended with observation for untoward effects of discontinuing study treatment. After study treatment ends, renewed consent will be requested for continued follow-up clinic visits at 3, 6, 9, 12, 16, 20, 24, and 30 months after the end of study treatment, and every six months thereafter for up to five years.